Layered automotive catalytic composite

ABSTRACT

This invention relates to a catalytic composite for treating an exhaust gas comprising a first support which is refractory inorganic oxide having dispersed thereon at least one noble metal component and having dispersed immediately thereon an overlayer comprising at least one oxygen storage component and optionally a second support which is a refractory inorganic oxide. The first support may be selected from the group consisting of alumina, silica, titania, zirconia and aluminosilicates with alumina being preferred. Additionally, the noble metal component may be selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component is an oxide of a metal which includes cerium, iron, nickel, cobalt lanthanum, neodymium, praesodymium, etc. and mixtures thereof. Cerium oxide is a preferred oxygen storage component. Finally, the second support may be selected from the group consisting of alumina, silica, titania, zirconia and aluminosilicates, with alumina preferred. This invention also relates to a process for minimizing the content of hydrogen sulfide in an automotive exhaust gas which comprises contacting said exhaust gas with a catalyst composite comprising a first support which is a refractory inorganic oxide having dispersed thereon at least one noble metal component and having dispersed immediately thereon an overlayer comprising at least one oxygen storage component and optionally a second support which is refractory inorganic oxide.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of prior copendingapplication Ser. No. 088,745, filed Aug. 24, 1987, now abandoned.

BACKGROUND OF THE INVENTION

Gaseous waste products resulting from the combustion ofhydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbonmonoxide, hydrocarbons and nitrogen oxides as products of combustion orincomplete combustion, and pose a serious health problem with respect topollution of the atmosphere. It is well known to use catalyticcomposites to simultaneously convert carbon monoxide, hydrocarbon andnitrogen oxide pollutants to innocuous gases. In order to achieve thesimultaneous conversion of said pollutants, a catalytic composite(commonly called a three component control catalyst) is ordinarily usedin conjunction with an air/fuel ratio control means which functions inresponse to a feedback signal from an oxygen sensor in the engineexhaust systems. The air-to-fuel ratio control means is typicallyprogrammed to provide fuel and air to the engine at a ratio at or nearthe stoichiometric balance of oxidants and reductants in the hot exhaustgases at engine cruising conditions, and to a stoichiometric excess ofreductants during engine startup and at engine acceleration conditions.The result is that the composition of the exhaust gases with which thecatalyst is contacted fluctuates almost constantly, such that conditionsto which the catalyst is exposed are alternatively net-reducing (fuelrich) and net-oxidizing (fuel lean). A catalyst for the oxidation ofcarbon monoxide and hydrocarbons and the reduction of nitric oxide mustbe capable of operating in such a dynamic environment.

The exhaust gas also contains other components such as sulfur oxides,phosphorus and zinc compounds which are known catalyst poisons. Thesulfur oxides present in the exhaust stream can react with the catalystto form other products. For example under fuel lean (net-oxidizing)conditions, sulfur dioxide (SO₂) reacts with oxygen (O₂) over thecatalyst to form sulfur trioxide (SO₃) which is then converted tosulfates (SO₄ ⁼) by reaction with water. Under fuel rich (net-reducing)conditions the SO₂ reacts with hydrogen (H₂) to form hydrogen sulfide(H₂ S). The formation of H₂ S is particularly objectionable because ofits strong odor.

In addition to the formation of H₂ S over a noble metal catalyst, astorage phenomenon has also been observed. This storage phenomenon hasbeen documented in the literature, G. J. Barnes and J. C. Summers,"Hydrogen Sulfide Formation Over Automotive Oxidation Catalysts,"Society of Automotive Engineers, Paper No. 750093. The experimentersshowed that sulfur accumulated on noble metal catalysts under bothoxidizing and reducing atmospheres. For example, under oxidizingconditions the sulfur is typically stored as sulfates (SO₄ ⁼) which isconverted to H₂ S under reducing conditions.

Although this phenomenon has been recognized for many years, the problemwhich it generates, i.e. unpleasant odor, was relatively minor and wasnot of much concern until recently. In the past few years automotivecatalyst technology has improved so that the catalysts are much moreactive than previous catalysts. Part of this improvement has beenachieved by increasing the content of the oxygen storage componentpresent in the catalytic composite. The most commonly used oxygenstorage components are the rare earths. Unfortunately, the rare earthsappear to increase the storage of sulfur during fuel lean operation, andwhen release occurs the concentration of hydrogen sulfide is much largerthan would have been anticipated, based on the sulfur content of thefuel. Consequently, the resultant odor is quite noticeable and many moredrivers are offended by the increased hydrogen sulfide odor.

Since the odor has become more noticeable and objectionable, a needexists to minimize the hydrogen sulfide emissions from catalyst equippedautomobiles. The instant invention cures this problem by providing acatalytic composite in which the oxygen storage component is separatedfrom the noble metal component. This is accomplished by depositing thenoble metal component on a first support which is a refractory inorganicoxide and then depositing a layer consisting of an oxygen storagecomponent and optionally a second support which is a refractoryinorganic oxide immediately thereover.

The prior art, U.S. Pat. No. 3,873,469, does disclose a layeredcatalytic composite. Additionally, Japanese Public Disclosures 71537/87and 71538/87 disclose a catalytic composite consisting of a ceramiccarrier having dispersed thereon a catalytic layer containing one ormore of Pd, Pt and Rh and an alumina layer containing one or more oxidesof Ce, Ni, Mo and Fe. However, the stated advantage of the 71537invention is that the oxides, which are oxygen storage components, renewthe catalytic surface. This is accomplished by having the oxygen storagecomponent in contact with the catalytic surface.

In contrast to this prior art, the present invention separates thecatalytic layer from the oxygen storage component. This separation is incontravention to the prior art which states that intimate contact of theoxygen storage component with the catalytic metal is required in orderfor the oxygen storage component to be effective. Therefore, the instantinvention differs from the prior art in that the oxygen storagecomponent is separated from the catalytic or noble metal component.Additionally, the result of this difference is to minimize the formationof H₂ S over a catalytic composite, even though said catalytic compositecontains the same or greater amount of an oxygen storage component as aconventional catalyst, a result quite distinct from that which couldreasonably be expected from the teachings of the prior art.

SUMMARY OF THE INVENTION

This invention relates to a catalytic composite and a process for usingsaid composite to treat an exhaust gas from an internal combustionengine. The catalytic composite comprises a first support which is arefractory inorganic oxide having dispersed thereon at least one noblemetal component selected from the group consisting of platinum,palladium, rhodium, ruthenium and iridium and having dispersedimmediately thereon an overlayer comprising at least one oxygen storagecomponent and a second support which is a refractory inorganic oxide.

Accordingly, one specific embodiment of the invention comprises acatalytic composite for treating an exhaust from an internal combustionengine, said catalytic composite comprising alumina having dispersedthereon platinum and rhodium and having dispersed immediately thereon alayer consisting of cerium oxide and alumina.

It is another embodiment of this invention to provide a process forminimizing the content of hydrogen sulfide in an automotive exhaust gaswhich comprises contacting said exhaust gas with a catalytic compositecomprising a first support which is a refractory inorganic oxide havingdispersed thereon at least one noble metal component selected from thegroup consisting of platinum, palladium, rhodium, ruthenium and iridiumand having dispersed immediately thereon an overlayer comprising atleast one oxygen storage component and a second support which is arefractory inorganic oxide.

Other objects and embodiments will become more apparent after a moredetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As heretofore indicated, the present invention relates to a catalyticcomposite and a process for minimizing the content of hydrogen sulfidein an automotive exhaust using said catalytic composite. The catalyticcomposite comprises a first support which is a refractory inorganicoxide having dispersed thereon at least one noble metal componentselected from the group consisting of platinum, palladium, rhodium,ruthenium and iridium and having dispersed immediately thereon anoverlayer comprising at least one oxygen storage component and a secondsupport which is a refractory inorganic oxide.

Accordingly, considering the first support utilized in the presentinvention, this first support includes but is not limited to alumina,silica, titania, zirconia, aluminosilicates, and mixtures thereof withalumina being preferred. When alumina is the desired first support, anyalumina which is well known in the art, such as described in U.S. Pat.No. 4,492,769, may be used.

The first support of the instant invention can be used in anyconfiguration, shape, or size which exposes the noble metal componentdispersed thereon to the gas to be treated. The choice of configuration,shape and size of the support depends on the particular circumstances ofuse of the catalytic composite of this invention. One convenient shapewhich can be employed is particulate form. In particular, the firstsupport can be formed into shapes such as pills, pellets, granules,rings, spheres, etc. The particulate form is especially desirable wherelarge volumes of catalytic composites are needed, and for use incircumstances in which periodic replacement of the catalytic compositemay be desired. In circumstances in which less mass is desirable or inwhich movement or agitation of particles of said first support mayresult in attrition, dusting and resulting loss of disposed metals orundue increase in pressure drop across the particles, a monolithicstructure is preferred.

Thus, a specific example of the present invention is alumina sphereswhich may be continuously manufactured by the well known oil drop methodwhich comprises: forming an alumina hydrosol by any of the techniquestaught in the art and preferably by reacting aluminum metal withhydrochloric acid; combining the resulting hydrosol with a suitablegelling agent; and dropping the resultant mixture into an oil bathmaintained at elevated temperatures. The droplets of the mixture remainin the oil bath until they set and form hydrogel spheres. The spheresare then continuously withdrawn from the oil bath and typicallysubjected to specific aging and drying treatments in oil and anammoniacal solution to further improve their physical characteristics.The resulting aged and gelled particles are then washed and dried at arelatively low temperature of about 149°-205° C. and subjected to acalcination procedure at a temperature of about 455°-705° C. for aperiod of about 1 to about 20 hours. This treatment effects conversionof the alumina hydrogel to the corresponding crystalline gamma-alumina.See the teachings of U.S. Pat. No. 2,620,314 for additional details.

If it is desirable to employ a monolithic form, it is usually mostconvenient to employ the first support as a thin film or coatingdeposited on an inert carrier material, which provides the structuralsupport for said first support. The inert carrier material can be anyrefractory material such as a ceramic or metallic material. It ispreferred that the carrier material be unreactive with the first supportand not be degraded by the gas to which it is exposed. Examples ofsuitable ceramic materials include sillimanite, petalite, cordierite,mullite, zircon, zircon mullite, spodumene, alumina-titanate, etc.Additionally, metallic materials may be used. Metallic materials whichare within the scope of this invention include metals and alloys aredisclosed in U.S. Pat. No. 3,920,583 which are oxidation resistant andare otherwise capable of withstanding high temperatures.

The carrier material can best be utilized in any rigid unitaryconfiguration which provides a plurality of pores or channels extendingin the direction of gas flow. It is preferred that the configuration bea honeycomb configuration. The honeycomb structure can be usedadvantageously in either unitary form, or as an arrangement of multiplemodules. The honeycomb structure is usually oriented such that gas flowis generally in the same direction as the cells or channels of thehoneycomb structure. For a more detailed discussion of monolithicstructures, refer to U.S. Pat. No. 3,785,998 and U.S. Pat. No.3,767,453.

The first support may be deposited on said solid monolithic carrier byany conventional means known in the art. One convenient method is bydipping the solid carrier into a slurry of said first support. As anexample when alumina is the desired first support, the preparation of aslurry from alumina is well known in the art and consists of adding thealumina to an aqueous solution of an acid such as nitric, hydrochloric,sulfuric, etc. The concentration of acid in said aqueous solution is notcritical but is conveniently chosen to be about 1 to about 4 weightpercent. Enough alumina should be added to said aqueous acid solutionsuch that the specific gravity of the final slurry is in the range ofabout 1.1 to about 1.9. The resultant mixture is ball milled for about 2to 24 hours to form a usable slurry which can be used to deposit a thinfilm or coating onto the monolithic carrier.

The actual coating procedure involves dipping the monolithic carrierinto said first support slurry, blowing out the excess slurry, dryingand calcining in air at a temperature of about 350° to about 800° C. forabout 1 to about 2 hours. This procedure can be repeated until thedesired amount of first support on said monolithic carrier is achieved.It is preferred that the first support, such as alumina, be present onthe monolithic carrier in amounts in the range from about 28 g ofsupport per liter of carrier volume to about 355 g of support per literof carrier volume, where the volume is measured by the exteriordimensions of the monolithic carrier structure.

A second feature of the catalytic composite of this invention is thatsaid first support has dispersed thereon at least one noble metalcomponent selected from the group consisting of platinum, palladium,rhodium, ruthenium and iridium. The noble metal component may bedispersed on said first support by several methods well known in the artincluding coprecipitation, cogellation, ion exchange or impregnation. Ofthese methods one convenient method of dispersing said noble metalcomponent on said first support is impregnation of said first supportwith an aqueous solution of a decomposable compound of said noble metal,drying and calcining in air to give a fine dispersion of said noblemetal on said first support.

Illustrative of the decomposable compounds of said noble metals arechloroplatinic acid, ammonium chloroplatinate, hydroxy disulfiteplatinum (II) acid, bromoplatinic acid, platinum trichloride, platinumtetrachloride hydrate, platinum dichlorocarbonyl dichloride,dinitrodiamino platinum, sodium tetranitroplatinate, rhodiumtrichloride, hexaminerhodium chloride, rhodium carbonylchloride, rhodiumtrichloride hydrate, rhodium nitrate, sodium hexachlororhodate, sodiumhexanitrorhodate, chloropalladic acid, palladium chloride, palladiumnitrate, diaminepalladium hydroxide, tetraaminepalladium chloride,hexachloroiridate (IV) acid, hexachloroiridate (III) acid,dichlorodihydrooxoiridate (III) acid, ammonium hexachloroiridate (III),ammonium aquohexachloroiridate (IV), tetraaminedichloroiridate (III)chloride, and aquotetraammineiridate (III) chloride, rutheniumtetrachloride, hexachlororuthenate, and hexamineruthenium chloride. Ofthe compounds enumerated above, the following are preferred fordispersing the desired noble metal: chloroplatinic acid, rhodiumchloride, chloropalladic acid, hexachloroiridate (IV) acid andhexachlororuthenate.

For three component control operation, it is desirable that thecatalytic composite contain a combination of rhodium and platinum,palladium or mixtures thereof. Specific combinations include platinumand rhodium, palladium, platinum and rhodium, palladium and rhodium.However, under certain circumstances, e.g. when control of nitric oxideis not necessary, it is undesirable (from an economic consideration) forthe catalytic composite to contain rhodium. In that case it is desirablefor the catalytic composite to contain platinum, palladium and mixturesthereof.

When more than one noble metal is desired, the metals can be in a commonaqueous solution or in separate aqueous solutions. When separate aqueoussolutions are used, impregnation of said first support with said noblemetal solutions can be performed sequentially in any order. Finally,hydrogen chloride, nitric acid or other suitable materials may be addedto said solutions in order to further facilitate the uniformdistribution of the noble metal components throughout said firstsupport.

When said first support is to be deposited on a solid monolithiccarrier, said first support may be impregnated with said aqueous noblemetal solution either before or after the first support is deposited onsaid solid monolithic carrier. Of the two procedures, it is moreconvenient to impregnate the noble metal onto the first support after ithas been deposited on said solid monolithic carrier.

It is desirable that the noble metal be present on said first support ina concentration ranging from about 0.01 to about 4 weight percent ofsaid first support. Specifically, in the case of platinum and palladiumthe range is from about 0.1 to about 4 weight percent. In the case ofrhodium, ruthenium and iridium, the range is about 0.01 to about 2weight percent. If both platinum and rhodium are present, the ratio ofthe platinum to rhodium content is from about 2:1 to about 20:1platinum:rhodium. The same is true if palladium and rhodium are present.

A third feature of the catalytic composite of this invention is anoverlayer comprising at least one oxygen storage component andoptionally a second support which is a refractory inorganic oxide. Thisoverlayer is dispersed immediately thereover said first supportcontaining at least one noble metal component. The oxygen storagecomponent is an oxide of a metal selected from the group consisting ofiron, nickel, cobalt and the rare earths, with the rare earths beingpreferred. Illustrative of the rare earths contemplated as within thescope of the invention are cerium, lanthanum, neodymium, europium,holmium, ytterbium, praesodymium, dysprosium, and mixtures thereof.Specific preferred rare earths are cerium, lanthanum, and mixtures ofcerium and lanthanum. Additionally, if a second support is present insaid overlayer, said second support may be selected from the groupconsisting of alumina, silica, titania, zirconia, aluminosilcates, andmixtures thereof, with alumina being preferred.

The overlayer which comprises said oxygen storage component may beapplied to said first support by means known in the art such as using acolloidal dispersion of the oxygen storage component, impregnating withmetal compounds of the oxygen storage components that do not penetrateinto the micropores of said first support, etc. Further, when saidoverlayer comprises an oxygen storage component and a second support,said oxygen storage component may be dispersed onto said second supportor a solid form of said oxygen storage component may be mixed with saidsecond support. Although it is not essential that the overlayer containa second support, it is preferred that a second support be present formonolithic applications.

Thus, one example of a method to disperse an oxygen storage componentonto said second support is to impregnate the refractory inorganic oxidewith an aqueous solution of a decomposable compound of said oxygenstorage component, drying and calcining in air the resultant mixture togive a second support which contains an oxide of said oxygen storagecomponent. Examples of water soluble decomposable oxygen storagecomponents which can be used include but are not limited to ceriumacetate, lanthanum acetate, neodymium acetate, europium acetate, holmiumacetate, yttrium acetate, praesodymium acetate, dysprosium acetate, ironacetate, cobalt acetate, nickel acetate, cerium nitrate, lanthanumnitrate, neodymium nitrate, europium nitrate, holmium nitrate, yttriumnitrate, praesodymium nitrate, dysprosium nitrate, iron nitrate, cobaltnitrate, nickel nitrate, cerium chloride, lanthanum chloride, neodymiumchloride, europium chloride, holmium chloride, yttrium chloride,praesodymium chloride, dysprosium chloride, iron chloride, cobaltchloride, nickel chloride.

Accordingly, in one specific example an appropriate amount of alumina isadded to an aqueous solution of cerium acetate. This mixture is thendried and calcined in air at a temperature of about 400° to about 700°C. for a time of about one to three hours. This results in the formationof cerium oxide which is well dispersed throughout the alumina.

Alternatively, a solid form of said oxygen storage component is mixedwith the appropriate amount of said second support. After mixing, ahomogeneous mixture of the two solids is obtained. The criteria requiredof the solid form of said oxygen storage component are that (1) it beinsoluble in water and in the acid/water solution used to prepare aslurry as described above, and (2) if the solid is not the metal oxidethat said solid decompose to the oxide upon calcination in air. Examplesof these insoluble solids include cerium sulfate, lanthanum sulfate,neodymium sulfate, europium sulfate, holmium sulfate, yttrium sulfate,iron sulfate, cobalt sulfate, nickel sulfate, cerium oxalate, lanthanumoxalate, neodymium oxalate, europrium oxalate, holmium oxalate, yttriumoxalate, iron oxalate, nickel oxalate, cobalt oxalate, cerium oxide,lanthanum oxide, neodymium oxide, europium oxide, holmium oxide, yttriumoxide, iron oxide, nickel oxide, cobalt oxide with the oxides beingpreferred. Thus a specific example consists of adding cerium oxide to analumina powder.

When the catalytic composite is to be used in the form of a solidmonolithic carrier, one method of applying said overlayer is to preparea slurry of said oxygen storage component and optionally a secondsupport and apply said slurry immediately over the first supportcontaining at least one noble metal which has been deposited on saidmonolithic support; said overlayer may be applied in the same manner asdescribed above for the first support.

It is desirable to apply an overlayer such that the concentration ofsaid oxygen storage component is about 2 to about 75 weight percent ofsaid first support and preferably from about 20 to about 70 weightpercent. Additionally, when the overlayer also contains a secondsupport, said second support is present in a concentration of 0 to 80weight percent of said overlayer. More preferably, the concentration ofsaid second support is about 30 to about 70 weight percent of saidoverlayer.

When particulate form is desired, said oxygen storage component may beseparated from the noble metals by controlling the penetration depth ofthe noble metals into the interior of the particulates. For example, thenoble metals can be made to penetrate into the interior of the spheresor other particulate form by means well known in the art such as theadding of chloride ions or a carboxylic acid to the impregnatingsolution. Subsequently, the oxygen storage component may be placed on ornear the surface of the spheres or particulates by means as describedabove, i.e., using a colloidal dispersion of said oxygen storagecomponent or using oxygen storage compounds which do not penetrate intothe micropores of the spheres.

Accordingly, one specific example consists of alumina spheres which havebeen impregnated with platinum and rhodium. These spheres are thenimpregnated with a solution of cerium nitrate. The spheres are thendried and calcined in air.

Thus, the resultant catalytic composite is characterized by the noblemetal component being separated from the rare earth component. Asmentioned above this configuration of the noble metal and oxygen storagecomponent is contrary to the prior art which teaches that an intimatemixture of the noble metal and oxygen storage component is necessary inorder for the catalytic composite to effectively treat an automotiveexhaust gas. It has been determined, however, that when the oxygenstorage component is separated from the noble metals, a larger amount ofoxygen storage component is necessary to obtain an equivalent catalyticcomposite to one in which the oxygen storage component is intimatelymixed with the noble metal component. But the requirement of additionaloxygen storage component over a conventional catalytic composite isoffset by the ability of the instant catalytic composite to minimize theamount of H₂ S produced. Therefore, the catalytic composite of thepresent invention shows unexpected results over the prior art.

Another embodiment of the instant invention is a process for minimizingthe H₂ S formation over a catalytic composite used for treating anautomotive exhaust. The process comprises contacting said automotiveexhaust with the catalytic composite described heretofore. A mechanismof how H₂ S is formed over a catalytic composite and how the instantprocess minimizes the H₂ S formation follows. It is to be understoodthat this mechanism is presented by way of illustration and is notintended to limit the broad scope of the invention.

Gasoline fuel typically contains about 0.005 to about 0.7 weight percentsulfur, usually as organic sulfur compounds. During the combustionprocess these sulfur compounds are converted to gaseous sulfur compoundssuch as SO₂ and SO₃. In order to completely combust the gasoline fuel,at least a stoichiometric weight amount of air is required. For exampleif the gasoline is indolene, the stoichiometric weight ratio is 14.56:1.of air:fuel. Using this ratio, therefore, one obtains that the gaseoussulfur compound concentration in the exhaust gas may range from about 3to about 480 ppm.

During the time that the exhaust gas is stoichiometric or net oxidizing,certain components of the catalyst are capable of reacting with thegaseous sulfur compounds in the exhaust gas (SO₂ and SO₃) and with O₂ inthe exhaust gas to form stable sulfates. For example, alumina would beexpected to form sulfates of aluminum such as Al₂ (SO₄)₃ at temperaturesbelow about 400° C. and at a gaseous sulfur oxides (SO_(x))concentration of 20 ppm; cerium oxide will similarly form sulfates ofcerium such as Ce₂ (SO₄)₃ at the same gaseous SO_(x) level attemperatures below about 500° C., while lanthanum oxide will formsulfates of lanthanum at the same SO_(x) level but at temperatures belowabout 700° C.

The resultant sulfates formed on the catalytic composite at theconditions described above are unstable under fuel rich conditions.Therefore, when the air/fuel ratio becomes fuel rich, the solid sulfateswill begin to decompose with the subsequent formation of hydrogensulfide (H₂ S), which will be emitted from the exhaust at concentrationsthat may be noxious. The reason that the concentrations of H₂ S may beexcessively high is that large amounts of sulfates can be stored on acatalytic composite under periods of stoichiometric or fuel leanoperations and then released during periods of fuel rich operation.

Since the exhaust temperature is normally above 400° C., the majority ofthe H₂ S which is released comes from the formation of rare earthsulfates. Thus, it has been found that using a catalytic composite thatdoes not contain any rare earth oxides minimizes the formation andrelease of H₂ S. However, the ability of a catalytic composite that doesnot contain a rare earth oxide to treat an automotive exhaust isinferior to a catalytic composite that does contain a rare earth oxide.

However, the present invention solves this problem by separating therare earth oxide from the noble metals. It appears that by separatingthe oxygen storage component from the noble metals, the correspondingmetal sulfates are not formed as easily, thereby reducing the amount ofH₂ S which is formed. Additionally, any SO₂ or SO₃ species which may bestored on the oxygen storage components are not as easily converted toH₂ S. The reason for this is that the conversion of SO₂ or SO₃ to H₂ Sunder fuel rich conditions requires a catalyst. Since the SO₂ or SO₃ isreleased from an overlayer above the noble metals, as the SO₂ or SO₃ arereleased, they will be swept away by the exhaust gas and not have achance to diffuse where the noble metals are located and form H₂ S. Theend result is that H₂ S formation is minimized.

In order to more fully illustrate the advantages to be derived from theinstant invention, the following examples are set forth. It is to beunderstood that the examples are only by way of illustration and are notintended as an undue limitation on the broad scope of the invention assetforth in the appended claims.

EXAMPLE I

This example describes a test developed to measure hydrogen sulfideemissions. The test consisted of two parts: storage of sulfur compoundsand hydrogen sulfide release. The conditions for these two parts of thetest are presented in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Test Condition     Storage  Release                                           ______________________________________                                        Inlet Temperature (°C.)                                                                   515      492                                               Air/Fuel Ratio     14.80    13.10                                             (Stoichiometry = 14.56)                                                       GHSV(hr.sup.-1)    63,000   63,000                                            Duration (min)     60       15                                                Fuel Sulfur Level (ppm)                                                                          120      120                                               ______________________________________                                    

The catalytic composite was first exposed to the storage condition tostoresulfate. Then the air/fuel ratio was changed to the releasecondition. Exhaust samples were collected in scrubbers and analyzed forH₂ S using a colorimetric method established by the EnvironmentalProtection Agency (EPA). Details on the H₂ S analysis method areavailable in EPA interim report EPA-600/2-80-068 which is available fromthe EPA. Samples were taken during the following time intervals: 0-1,1-2, 2-3, 3-4, 9-10, and 14-15 minutes after switching to the releasecondition.

Controlling the release of H₂ S during the first two minutes of fuelrich operation is most important because fuel rich modes of operationtypically do not last substantially longer than two minutes.Additionally,the H₂ S emissions are most objectionable when released aslarge spikes. These two criteria should be used to evaluate thecatalytic composites herein described.

EXAMPLE II

The conventional catalytic composite was prepared by the followingmethod. In a beaker, 5000 grams of pseudo-boehmite alumina and 13,562grams of a solution of cerium acetate was mixed, which solutioncontained 7 weight percent cerium. The resultant mixture was stirred for30 minutes, transferred to a shallow tray, dried for 4 hours at 150° C.and finally calcined at 600° C. for 1 hour. The calcinedalumina/ceriumoxie powder was next stirred into a container whichcontained 5.33 liters of water and 48 mL of concentrated nitric acid(HNO₃). This mixture was ball milled for 4 hours.

An oval shpaed cordierite monolith with a minor axis of 3.18 inches(8.08 cm), a major axis of 6.68 inches (16.97 cm), a length of 4.5inches (11.43cm) and having 400 square channels per square inch offacial area was dipped into the above described slurry. After dipping,the excess slurry was blown out with an air gun. The slurry coatedmonolith was calcined forabout 1 hour at 540° C. The above describeddipping, blow-out and calcining steps were repeated until the monolithcontained 128 g of coating per liter of monolith volume.

Next the platinum and rhodium metals were impregnated onto theabove-described washcoated monolith. The above-described monolith wasdipped into an aqueous solution containing 10 mg of chloroplatinic acid(29.5 weight % Pt) per gram of solution and 10 mg of rhodium chloride(9.8weight % rhodium) per gram of solution and 5 weight percent sugar.After dipping, the impregnated monolith was dried and calcined for aboutone hour at 540° C. This catalytic composite was designated CatalystA.The calculated composition of Catalyst A in units of g/liter was:Pt=1.5; Rh=0.3; Ce=23.3.

EXAMPLE III

A catalytic composite of the present invention was prepared as follows.To a container which contained 5.3 liters of water and 48.0 mL ofconcentrated nitric acid (HNO₃), there were added 5,000 grams of deltaalumina. This mixture was ball milled for 24 hours.

An oval shaped cordierite monolithic carrier with a minor axis of 3.18inches (8.08 cm), a major axis of 6.68 inches (16.97 cm), a length of4.5 inches (11.43 cm) and having 400 square channels per square inch offacialarea was dipped into the above described slurry. After dipping,the excess slurry was blown out with an air gun. The slurry coatedmonolith was calcined for about 1 hour at 540° C. The above describeddipped, blow-out and calcining steps were repeated until the monolithcontained 128 g of coating per liter of monolith volume.

Next the platinum and rhodium metals were impregnated onto theabove-described washcoated monolith. The above-described monolith wasdipped into an aqueous solution containing 10 mg of chloroplatinic acid(29.5 weight % Pt) per gram of solution, 10 mg of rhodium chloride (9.8weight percent rhodium) per gram of solution, and 5 weight percentsugar. After dipping, the impregnated monolith was calcined for aboutone hour at540° C.

Finally, 26,800 grams of cerium acetate solution (7 weight percentcerium) and 5,000 grams of pseudo-boehmite alumina were mixed in acontainer and processed as in Example II. The resultant powder was usedto prepare a slurry as in Example II. This slurry was applied to themonolithic carrieras described above such that the carrier contained 85g of a ceria/alumina overlayer per liter of monolith. This catalyticcomposite was designated Catalyst B. The calculated composition ofcatalyst B in units of g/liter was: Pt=1.5; Rh=0.3; Ce=35.3.

EXAMPLE IV

Catalysts A and B were tested according to the procedure set forth inExample I and the results are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                                    H.sub.2 S Concentration During                                                Release Intervals (mins.)                                                     (mgH.sub.2 S/m.sup.3 of gas scrubbed)                             Catalyst I.D. 0-1    1-2       2-3  14-15                                     ______________________________________                                        Catalyst A    205    80        52   2                                         (conventional)                                                                Catalyst B     32    21        15   9                                         ______________________________________                                    

The data clearly indicate that the catalyst of the instant invention(Catalyst B) minimizes the amount of H₂ S which is released.Additionally, the data show that the amount of H₂ S which Catalyst Breleased during the first two minutes of the test is more than fivetimes less than the conventional catalyst. Controlling the release of H₂S during the first two minutes of fuel rich operation is most importantbecause fuel rich modes of operation typically do not last substantiallylonger than two minutes. Additionally, the H₂ S emissions are mostobjectionable when released as large spikes such as exhibited by theconventional catalyst (Catalyst A).

EXAMPLE V

The following tests were conducted in order to show that the catalyst ofthe instant invention has at least equivalent ability to treat anexhaust gas from an internal combustion engine. Fresh samples ofCatalyst A and Catalyst B were prepared as per Examples II and IIIrespectively. Each catalyst was mounted in a converter and eachconverter was placed in the exhaust stream from one bank of a V-8gasoline fueled engine. The engine was operated according to thefollowing cycle.

The engine used for this durability cycle was a Ford 5.0 L V-8 engineequipped with duel throttle body fuel injector. The durability cycleconsisted of a 60 second cruise mode and a 5 second fuel cut mode.During the cruise mode, the engine operated a stoichiometry while duringthe fuelcut mode, the engine operated at a fuel lean condition thatincluded a temperature and an oxygen spike. The fuel cut mode isachieved by breakingthe circuit between one of the fuel injectors andthe Electronic Engine Control. The engine speed and load on the enginewas adjusted to give an exhaust gas temperature of 850° C. during thecruise mode and 800° C. during the fuel cut mode. This cycle wasrepeated for 25 hours.

After Catalysts A and B were exposed to the durability cycle asdescribed above, they were evaluated using an engine dynamometerperformance test. The test involved evaluating the catalyst at sevendifferent air/fuel (A/F) ratio points (14.71, 14.66, 14.61, 14.56,14.51, 14.46 and 14.41) atan inlet temperature of 450° C. At each A/Fpoint, the air/fuel was oscillated plus or minus 0.1 A/F at one Hertzfrequency. Conversions of hydrocarbon, carbon monoxide and nitric oxideswere calculated at each A/Fand then an integral performance conversionwas obtained by averaging all the conversions.

Additionally, Catalysts A and B were evaluated using a test whichconsistedof a continuous temperature traverse at an A/F ratio ofapproximately 14.55. During this test the temperature of the exhaust gasgoing into the converter was continuously varied from 200° C. to 460° C.byvarying the heat transfer rate of a stainless steel heat exchanger.Conversion of hydrocarbon, carbon monoxide and nitric oxides werecalculated as a function of temperature. The time required to reach 50%conversion is a common criterion used to evaluated catalytic composites(referred to as light off performance) and is reported here. The resultsof these evaluations are present in Table 3.

                  TABLE 3                                                         ______________________________________                                                 Temperature (°C.)                                                                   Integral                                                         Required to Reach                                                                          Performance                                                      50% Conversion                                                                             % Conversion                                            Catalyst I.D.                                                                            HC     CO       NO.sub.x                                                                           HC   CO     NO.sub.x                          ______________________________________                                        Catalyst A 355    365      326  89   60     68                                (Conventional)                                                                Catalyst B 355    364      316  88   61     68                                ______________________________________                                    

The data indicate that Catalysts A and B are at least equivalent after asevere durability test.

EXAMPLE VI

Fresh samples of Catalyst A and B prepared as described in Examples IIand III respectively were placed in a converter and tested according tothe temperature traverse test of Example V. These results are presentedin Table 4.

                  TABLE 4                                                         ______________________________________                                                   Temperature (°C.) Required                                             to Reach 50% Conversion                                            Catalyst I.D.                                                                              HC           CO     NO.sub.x                                     ______________________________________                                        Catalyst A   339          315    309                                          (Conventional)                                                                Catalyst B   322          301    279                                          ______________________________________                                    

These data show that the Catalyst of the instant invention (Catalyst B)hasa lower light-off temperature than the conventional catalyst. Thismeans that in actual use, i.e. when a vehicle is first started up,Catalyst B will be operational before Catalyst A. Thus, Catalyst B hasimproved activity over the catalyst of the prior art.

What is claimed is:
 1. A catalytic composite for treating an exhaust gasand minimizing the hydrogen sulfide content thereof, said catalyticcomposite comprising a first support which is a refractory inorganicoxide selected from the group consisting of alumina, silica, titania,zirconia, aluminosilicates and mixtures thereof, having dispersedthereon at least one noble metal component selected from the groupconsisting of platinum, palladium, rhodium, ruthenium, and iridiumsubstantially in the absence of an oxygen storage component and havingdispersed immediately thereon an overlayer separated from said noblemetal component comprising at least one oxygen storage component whichis an oxide of a metal selected from the group consisting of iron,nickel, cobalt and the rare earths and optionally a second support whichis a refractory inorganic oxide.
 2. The catalytic composite of claim 1where said first support is in the shape of pellets.
 3. The catalyticcomposite of claim 1 where said first support is deposited on a solidceramic or metallic honeycomb monolithic carrier.
 4. The catalyticcomposite of claim 1 where said first support is alumina.
 5. Thecatalytic composite of claim 1 where the noble metal component isplatinum, palladium or a mixture thereof, each metal present in aconcentration in the range of about 0.01 to 4 weight percent of saidfirst support and optionally rhodium in a concentration of 0.01 to 2weight percennt of said first support.
 6. The catalytic composite ofclaim 5 where the metals are a mixture of platinum and rhodium.
 7. Thecatalytic composite of claim 5 where the metals are a mixture ofpalladium and rhodium.
 8. The catalytic composite of claim 5 where themetals are a mixture of platinum, palladium and rhodium.
 9. Thecatalytic composite of claim 1 where said oxygen storage component insaid overlayer is present in a concentration of about 2 to about 75weight percent of said first support.
 10. The catalytic composite ofclaim 1 where said second support is present in a concentration of 0 to80 weight percent of sid overlayer.
 11. The catalytic composite of claim1 where said oxygen storage component is a rare earth oxide.
 12. Thecatalytic composite of claim 11 where said oxygen storage component islanthanum oxide.
 13. The catalytic composite of claim 11 where saidoxygen storage component is cerium oxide.
 14. The catalytic composite ofclaim 11 where said oxygen storage component consists of a mixture oflanthanum and cerium oxides.
 15. The catalytic composite of claim 1where said second support is selected from the group consisting ofalumina, silica, titania, zirconia and aluminosilicates.
 16. Thecatalytic composite of claim 15 where said second support is alumina.